1. Field of Invention
This invention relates to an antistatic coating composition in a conductive substrate and a process for forming the substrate. More specifically this invention relates to improved antistatic compositions which can be coated with a minimal salt content and with minimal or no added surfactant.
2. Description of Related Art
Permanent antistatic coating compositions are well known in the art. For example, U.S. Pat. Nos. 4,225,665; 4,701,403 and 4,859,570; disclose particularly useful antistatic layers for photosensitive elements. Auxiliary layers for use with antistatic coatings are described in U.S. Pat. Nos. 4,585,730; 4,940,555; 4,891,308 and 5,128,233.
The essential elements of a suitable antistatic coating include an electrically conductive polymer having integral groups which can be crosslinked; a crosslinking, or hardening compound; and an optional, yet preferred, plasticizer.
Widely used conductive polymers are those with a sulfonate, or sulfonic acid, as the charge carrying group and a carboxyl as the group to be crosslinked. These polymers have shown great utility as an antistatic coating yet several problems still exist in practice.
The polymer is typically available in the acid, or low pH form, or in the basic, or high pH form. In the basic form the carboxyl groups and sulfonate groups are predominantly ionic with a counter ion such as sodium or potassium. Due to the ionic charge these materials act as a surfactant in water and lower the surface energy. Several problems occur with basic, or ionic antistatic polymers.
The predominant problem with the basic, or high pH, form of the conductive polymer is the propensity to grow organisms. This problem is particularly bothersome when the solid conductive polymer must be stored for long periods of time.
Another problem with the basic form of the conductive polymer occurs when subsequent coats are added over the antistatic layer. It is well known in the art that the surface energy of a coating solution must be lower than the surface energy of the surface being coated. Therefore, a substrate with a high surface energy and a solution with a low surface energy is most desirable. Ionic antistatic polymers act as a surfactant, or soap, in solution which is highly desirable for coating. The problem occurs where subsequent coats are applied supra to the ionic antistatic polymer layer. The low surface energy is never lost and therefore any subsequent coating must be applied over a low energy surface which is undesirable. If other layers are to be coated supra to the antistatic layer, e.g., photosensitive layers, the surface energy of each successive layer away from the support must be lowered to facilitate coating. The surface energy of a coating solution is usually lowered by adding low molecular weight surfactants which migrate to the surface and lower the surface energy. Each successive layer must have higher amounts of surfactants to insure a surface energy lower than the previous layer. High levels of surfactant in the finished product are undesirable for a variety of reasons as known in the art.
Yet another problem with the basic, or high pH, antistatic polymers is a decrease in the effectiveness of the crosslinking reaction. It is well known in the art, and exemplified in U.S. Pat. No. 4,960,687 and 4,940,655, that preferred crosslinking reactions proceed most favorably at a pH below about 7.0. Addition of an acid forms mobile salts which often bloom to the surface and interfere with subsequent coatings. This problem is particularly evident with photosensitive layers which typically exhibit poor aging properties due to the salts formed.
Polymers with low salt content are typically the acid form wherein crosslinking groups, such as carboxyls, and charge carrying groups, such as sulfonates, are predominantly protonated. In the protonated form the polymer is less soluble in water and the surface tension of the solution is high due to the low number of ionic groups on the polymer. A solution with a high surface energy is difficult to coat as discussed previously. The problem of high surface energy has been circumvented in the art by the addition of surfactants and other ingredients which are well known in the coating art to lower surface tension and facilitate coating on a support as discussed in U.S. Pat. Nos. 5,094,909 and 5,098,822. Good quality coatings can be obtained in this manner and, in fact, the use of surfactants represents the standard procedure in the art. The surfactants are known to migrate to the surface of the material and, by so doing, lower the surface energy. The use of an acidic polymer coated with surfactants does decrease the effect of salts interfering with subsequent layers. The surfactants are known to migrate to the surface and the problem of low surface energy of the dried film still exist. Each subsequent coating solution must still have a lower surface energy than the layer directly below.
In the acid form the shelf life of the coating solution containing the polymer and crosslinking agent is also a critical problem. As mentioned previously, the activity of preferred crosslinking reactions increases as pH decreases. To counteract premature crosslinking, one must choose a less active crosslinker or use small batches to insure that material is not held long enough to begin crosslinking in the production kettle. Alternatively, the crosslinker can be added just prior to coating, as illustrated in the examples of U.S. Pat. No. 4,810,624. One skilled in the art understands the detrimental effect of polymer crosslinking in the production kettle.
The practitioner has therefore been placed in the unfortunate dilemma of choosing between two Undesirable modes of operation. A form of the polymer which is at a high pH, can be used but detrimental salts will be generated. Alternatively, an acid form of the polymer can be used but effective coating requires a surfactant and the solution has poor stability. The practitioner is also in a particular dilemma because either choice yields a coating wherein a surface energy of the dried coating is lower than desired and it is therefore difficult to apply subsequent layers as is often desired in the art.
There has been a long felt need in the art to provide an antistatic coating composition which has a low surface energy at the time of application and a higher surface energy after application and which is void of high levels of salts.